Recently, multipurpose functional fluids have been used as lubricating hydraulic fluids for various construction machinery and agricultural machinery. The multipurpose functional fluids are used for lubrication of power transmission devices (e.g., speed change gear and differential gear), elevating operation parts of machine tools and hydraulic systems (e.g., power steering system), and for lubrication of engine systems and wet type brake systems. Conventionally employed multipurpose functional fluids are prepared by compounding a mineral oil as a base oil with a detergent dispersant and an antioxidant and, if necessary, a viscosity index improver and other additives.
In recent years, lubricating fluids that leak from various machinery or waste lubricating oils cause problems of environmental pollution. For example, pollution of lake or sea is caused by two-stroke-cycle engine oils for motorboats. Waste pollution is caused by two-stroke-cycle engine oils for machine tools, such as those for chain saws. To cope with the pollutions, studies were made of the use of a fatty acid triglyceride having good biodegradability, e.g., a vegetable oil, as the base oil in place of the mineral oil. Studies are now being made of the use of two-stroke-cycle engine oils containing fatty acid triglyceride as the base oil.
Research has been made for the improvement of the biodegradability of hydraulic fluids. Studies in the field of hydraulic fluids include the use of fatty acid triglyceride having good biodegradability, such as a vegetable oil. See: SAE Technical Paper No. 910960, Apr. 9-10, 1991).
The present inventors have studied the use of a vegetable oil as the base oil in place of the mineral oil in accordance with technical information obtained so far. However, they have concluded that a multipurpose functional fluid using a vegetable oil as the base oil has insufficient stability, particularly in the oxidation stability.
The use, in lubricating products, of base oils other than mineral oils for their biodegradibility is well known. Synthetic ester base oils or naturally occurring ester mixtures such as vegetable oils provide lubricating ability and biodegradability to a lubricant formulation. A British patent, 1,189,541, published 29 Apr., 1970, teaches that esters of saturated aliphatic carboxylic acids using alcohols of at least five carbon atoms are usable as base oils for transmission lubricants. An European patent application, 0 103 884, published 28 Mar., 1984, teaches that esters of polyols and carboxylic acids are useful for transmission lubricants. There are other literature sources that teach the use of many different types of synthetic ester base oils as suitable for hydraulic fluid and transmission fluid base oils.
Among the large variety of ester base oils which have lubricity and biodegradability, however, there is a much smaller variety which can also be compatible with the additives used to provide the equipment performance lacking in the base oil itself. These additives provide oxidation resistance, rust prevention, brake chatter prevention, water tolerance, etc., which are not sufficient in the base oil alone. A class of such additives, useful for making multipurpose functional fluids, is not soluble in every ester base oil otherwise suitable based on its lubricity and biodegradability. In this invention, we have discovered those combinations of multipurpose functional fluid additive and ester base oil which also are compatible and allow a practical product to be formulated.